Lets start off with a basic look at the nitrogen cycle. In our aquariums, the nitrogen cycle (or cycling) begins with the presence of ammonia and bacteria within our tanks. That ammonia can be introduced in the form of ammonia itself (either directly by the hobbiest), indirectly through the introduction of live organisms (primarily fish) which produce waste that contains ammonia, or through the biological breakdown of a dead organism (such as a cocktail shrimp). The aerobic bacteria within the aquarium (either introduced by the hobbiest via "living" sand or rock, or the addition of "used" aquarium water, or having arrived on their own from spores in the air) begin the process of converting the ammonia (NH3) into nitrites (NO2-). Both ammonia and nitrites are toxic to fish at the high levels they are present at during the initial cycle; so introduction of living organisms should either wait until this portion of the cycle is complete or the hobbiests should very closely monitor the levels of both substances and continually do water changes to insure that these substances never reach toxic levels. While the toxicity of the toxins present may not be enough to cause instant death, research has shown that exposure at less then deadly levels is chronic and significantly shortens the life span of fish. The "cycle" is typically completed when the nitrites (NO2-) are further converted by aerobic bacteria into nitrates (NO3-), and a large scale water change is completed to remove the end products (nitrates) from the aquarium. While in truth the nitrogen cycle at this point is not complete, the portion of the "cycle" we are primarily interested in is considered complete. To truly complete the nitrogen cycle, we need to take this process farther by introducing biological methods to continue removing nitrates over the long term as living organisms continue to produce waste which must be processed through the nitrogen cycle. These methods of biological control can include macro algae which take up the nitrates and convert them into amino acids and anaerobic bacteria (contained deep within the pores of LR and within anaerobic areas of the sandbed (found in both deep and shallow sandbeds)) convert the nitrates (NO3-) into harmless nitrogen gas (N2). By harvesting the macro algae from your system you continue to remove the nitrates. Even then, the nitrogen cycle is not truly complete. Cyanobacteria (also known as blue-green algae) found within the saltwater aquarium is a nitrogen fixer, meaning that it has the ability to capture nitrogen gas (N2) released by the anaerobic bacteria and convert it into ammonia (NH3), Nitrite (NO2-), Nitrate (NO3-), and use that to promote their own growth.